Nanofibers have been shown to greatly improve the mechanical efficiency of both liquid and aerosol filtration, with only a modest increase in the pressure drop. The high surface area and fine pore size of nonwoven fabrics makes them ideal for selective permeability membranes. Nanofibers have been utilized in protective textiles as breathable barriers to liquid penetration, in nanoparticle filtration, heavy metal decontamination, and in fabrication of protein affinity membranes. Biocompatible and biodegradable nanofibers are considered essential for the future development of scaffolds in tissue engineering, as resorbable membranes that prevent post-surgery adhesions, as well as in the delivery of a number of agents, including antibiotics, DNA, or proteins.
The addition of inorganic particles to polymer fibers gives them additional functionality and often improves their mechanical properties. Polystyrene/clay and PE/carbon nanotube composite fibers have shown enhanced strength compared to the pure polymer fibers. Composite fibers with silver nanoparticles are studied intensely due to their antibacterial properties. Biocatalytic fibers are made by the inclusion of active enzymes in the fibers, or even by the incorporation of whole viruses, which can maintain their ability to infect bacteria. Embedding such composite fibers into protective clothing would impart additional functionalities besides superior softness and thermal insulation currently provided by microfibers.
Flexible electronics are another emerging area where nanofibers offer useful properties in sensing, miniaturization and integration with existing textiles. Sensing of chemical and optical signals has recently been demonstrated. Nanofibers can form one-dimensional heterojunctions, be assembled into field-effect transistors, and emit light by electroluminescence. Nanofibers can also make possible enhanced charge separation in photovoltaics. In all the above applications, high volume and low production cost are either essential for commercialization or could significantly benefit related applications.
Five general methods for the production of fibers with nanometer or single-micron diameters exist: drawing, phase separation, electrospinning, template synthesis and self-assembly. So far only three major technologies have the potential to produce these fibers on a commercial scale: melt blowing, splitting/dissolving of bicomponent fibers, and electrospinning. The first two techniques are based on mechanical drawing of melts and are well-established in high-volume manufacturing. In melt blowing polymers are extruded from dies and stretched to smaller diameters by heated, high velocity air streams. Bicomponent spinning involves extrusion of two immiscible polymers and two-step processing: (1) melt spinning the two polymer melts through a die with a “segmented pie” or “islands-in-the-sea” configuration, followed by solidification and (2) release of small filaments by mechanically breaking the fiber or by dissolving one of the components. For melt-processable polymers, this is the most commercially viable method due to its relatively high productivity. In individual cases both techniques have produced submicron diameter fibers (500 nm by melt blowing and 300 nm from bicomponent spinning), although commercial operations of melt blown fiber production are usually restricted to fibers thicker than 2 μm. The biggest disadvantage of these techniques is that they are limited to melt-processable polymers.
Many polymers of commercial interest, however, including acrylics and especially polymers that are biocompatible and biodegradable, are only processed from their solution. So far no commercial solution spinning method has been developed for creating nanofibers from such polymers. The two main types of solution spinning, dry-spinning and wet spinning, like melt spinning, also involve extrusion of the polymer through an orifice. In dry-spinning the polymer is then drawn through air at elevated temperature while the solvent evaporates. In wet-spinning the fiber is drawn in a coagulation bath. Wet spinning is the highest-volume technique for fiber production from solutions as it imposes few limitations on the polymer solvent, is often carried out at room temperature, and does not require handling of volatile solvents. Challenges exist for the production of smaller diameter fibers with wet-spinning. First, high pressures are needed for extrusion through smaller nozzles. Second, clogging of the small nozzles may occur, especially due to aggregation of particulate additives used to make composite fibers. Both of these limitations restrict the minimum wet-spun fiber diameter to 10-20 μm, yet bulk production of finer fibers from solution-processed polymers is highly desirable.
Electrospinning differs from melt or dry spinning by the fact that electrostatic rather than mechanical forces are used to draw the fibers. Among the three techniques mentioned, electrospinning can produce the smallest fibers (20-2000 nm in diameter), and is the only one that can produce sub-micron fibers from most polymers. Its recent popularity is partially due to numerous potential applications in protective barrier applications, tissue engineering and flexible electronics. However, its low production rate is a major disadvantage of this technique. New efforts to increase its throughput have included the use of multiple nozzles in parallel (multi jet electrospinning) though so far only technology provided by Elmarco, Inc., initiating fibers without nozzles from a thin film of polymer solution on a rotating drum, has the potential for commercial applications. According to the company literature, one of their commercial electrospinning units has a nanofiber production capacity of ˜0.4 g/min (˜0.1 g/min per spinning drum). For the wide commercialization of nanofibers there is an urgent need for a method capable of several orders of magnitude higher productivity.
Accordingly, an ongoing need remains for improved techniques for fabricating nano-scale diameter polymer fibers.